Along with the progress of the information processing technology, information digitalizing has been advanced in various fields. In addition to personal computers and servers that are conventional typical hardware items, it is now strongly required to store mass digital data in electric home appliances, audio and medical apparatuses, and others. In order to meet such requirements, magnetic disk drives (HDD) that are core of non-volatile file systems have been demanded rapidly to store such mass digital data more than ever. Realizing a large capacity magnetic disk drive means improvement of an area recording density, that is, a recording bit density on a medium. And in order to increase such an area recording density, the following three requirements must be satisfied simultaneously, concretely (1) realizing high SNR (realizing reduction of diameters of medium crystal grains and high sensitivity of the subject reading head), (2) keeping the heat resistance to demagnetization (increasing of a medium magnetic anisotropy energy Ku), (3) securing of write-ability (increasing the recording head magnetic field in narrow tracks).
The longitudinal recording having been adopted since the birth of the HDD uses a magnetic flux leaked slightly from a pole gap, so that the limit of the above (3) is especially exceeded at the area recording density of about 100 Gb/in2. The perpendicular recording method has been proposed to exceed the limit of the longitudinal recording. The perpendicular recording magnetizes the subject medium recording layer perpendicularly to the film surface and the recording principles differ from those of the conventional longitudinal recording media (IEEE Transactions on Magnetics, Vol. 20, No. 5, September 1984). Particularly, when a soft magnetic under layer is provided between a medium recording layer and a substrate and a single pole write sensor is used, a magnetic flux flows into the recording layer directly. Thus the recording magnetic field is larger than that of the longitudinal recording. In addition, because the perpendicular recording enables antiparallel array of adjacent magnetizations without making them face each other, thereby avoiding influences of demagnetizing fields. This is why it is expected to narrow the magnetization transition area significantly, thereby the linear recording density can be improved with high SNR. Furthermore, it is known that because the requirement for thinning the medium film is not so strong than that of the longitudinal recording, high thermal stability to demagnetization can be secured.
There have been proposed various types of magnetic heads conventionally to realize the perpendicular recording as described above. U.S. Pat. No. 5,073,836 and JP-A No. 62 (1993)-262213 disclose a configuration for disposing a read sensor between a main pole and a sub-pole or a configuration for disposing a read sensor in a multilayer magnetic material of the main pole. JP-A No. 2006-277834 discloses a magnetic head structure in which a ferromagnetic piece 24 referred to as a trailing shield is disposed closely to the main pole as shown in FIG. 17 to sharpen the recording magnetic field distribution at the trailing side of the main pole.